A. Field of Invention
The present invention relates, in general, to an electrostatic chuck for holding semiconductor wafers, especially silicon wafers, and to a method for fabricating such a device. More particularly, the present invention relates to a split-electrode electrostatic chuck in which electrical contact to the wafer is not required and to a sequence of operations used to manufacture that chuck.
B. Description of Related Art
In the manufacture of semiconductor devices, care must be taken when processing the semiconductor wafer. Processing treatments may involve directing charged particles toward the wafer. For example, selected areas of the wafer can have their conductivity type modified by implanting ions. As another example, wafers formed of such materials as Si, SiO.sub.2, Si.sub.3 N.sub.4, Al, W, No, Ti and the alloys of these metals and the like may be dry etched using a plasma etch, a sputter etch, or a reactive sputter etch.
Unfortunately, processing treatments involving the use of beams of charged particles generate thermal energy in the wafer. The problems caused by such thermal energy include expansion and local distortion of the wafer and, in the case of a plasma etch, for example, melting of the resist used for the mask. In order to avoid these problems, the heat generated must be dissipated quickly. Additional processing problems are created because the wafers usually are not perfectly flat; they have warps with lengths on the order of a few micrometers.
As is known in the art, improved processing of wafers can be achieved if the wafer is clamped substantially flat against a support base which is temperature controlled during treatment. By clamping the wafer, the number of points of contact between the wafer and the support is increased; therefore, the thermal conductivity between the wafer and support is enhanced, warps are corrected, and the contact area is extended. The improved heat transfer, whether to or from the wafer, enables better control of the temperature of the wafer and, hence, better process control.
Accordingly, a variety of chucks have been proposed to clamp the wafer during processing. Such chucks include mechanical types, vacuum types, and electrostatic types. Electrostatic chucks are particularly useful in the processes which produce semiconductor devices, however, because they can be used under vacuum conditions, do not need mechanical structure to hold the wafer, and can apply a uniform clamping force.
Two general types of electrostatic chucks are known. In the first type, an electrode is positioned on a support base. An insulator is placed over the electrode and, in turn, the wafer is placed over the insulator. Voltage is applied between the electrode and the wafer, creating an electrostatic force of attraction. Because the voltage is applied between the wafer and the electrode, electrical contact with the wafer is required. That requirement limits the material to be held, the wafer, to conductors, semiconductors, or materials at least covered with a conductive material on the surface. Thus, semiconductor wafers covered with an insulator such as an SiO.sub.2 film cannot be clamped using the single electrode type of electrostatic chuck.
The second type of electrostatic chuck includes an electrode split into two or more areas, or separate electrodes, which can be held at different potentials. The electrodes, typically planar, are positioned on a support base with a covering insulator and the wafer is placed on the insulator. Voltage is applied between the electrodes, generating a strong electric field which produces an attractive force even if the wafer is coated with an insulator; the wafer is always at a different potential with respect to some part of the split electrodes, regardless of what potential it may assume. Moreover, because the wafer is usually conductive, electrostatic capacities often exist between the wafer and the electrodes.
When the electrode is split into two or more surfaces, problems arise. The force of attraction is very sensitive to small deviations, on the order of a few microns, from the ideal, co-planar position of the surfaces--as well as to surface roughness and flatness in general. It is difficult to isolate the split electrode surfaces electrically yet maintain those surfaces in a flat, co-planar relationship.
Further, in practice, the electrostatic chuck may be part of an rf discharge apparatus. High frequency voltage (rf) is typically supplied through the support and electrode on which the wafer rests. Thus, an rf current flows through the support and electrode to ground or from an rf generator into the discharge through the support and electrode. That rf current may generate different rf potential drops across the support and electrode if the capacitive coupling between the two or more isolated parts of the electrode is too low.
With the above discussion in mind, it is one object of the present invention to provide a split electrode electrostatic chuck having a geometrical design which improves the strength and uniformity of the clamping force of attraction between the chuck and wafer. A second object is to assure that the surface of the split electrode is substantially a single, co-planar, flat, smooth surface. Also of advantage, and a further object, is a maximum thermal contact between the wafer and support.
Still another object of the present invention is to provide a high capacitance between the separate parts of the split electrode to minimize rf voltage drops. Another object is to assure the integrity of the insulator coating of the chuck during manufacture. A related object is to protect the insulating coating of the chuck from the processing treatment applied to the wafer.